Tropical Storm ISAAC

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TROPICAL STORM ISAAC DISCUSSION NUMBER 30
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
1000 AM CDT TUE AUG 28 2012
REPORTS FROM AIR FORCE AND NOAA HURRICANE HUNTER AIRCRAFT INDICATE
THE CENTRAL PRESSURE HAS DECREASED TO ABOUT 976 MB SINCE THE
PREVIOUS ADVISORY...BUT IT HAS LEVELED OFF OVER THE PAST FEW HOURS.
THE MAXIMUM 850 MB FLIGHT-LEVEL WIND OBSERVED HAS BEEN 89 KT...
WHICH EQUATES TO A SURFACE WIND OF ABOUT 71 KT. HOWEVER...
BIAS-ADJUSTED SFMR SURFACE WINDS HAVE ONLY BEEN AROUND 60-62 KT...
WHICH CORRELATES WELL WITH DROPSONDE BOUNDARY LAYER-DERIVED SURFACE
WINDS. AS SUCH...THE INTENSITY OF ISAAC IS BEING MAINTAINED AT 60
KT...WHICH IS JUST BELOW HURRICANE STATUS.
CENTER FIXES FROM THE AIRCRAFT INDICATE THAT ISAAC HAS BEEN WOBBLING
IN A GENERAL NORTHWESTWARD MOTION OF 310/09 KT. THERE REMAINS NO
SIGNIFICANT CHANGE TO THE PREVIOUS FORECAST TRACK OR REASONING.
ISAAC IS FORECAST TO MOVE INTO A WEAKNESS IN THE SUBTROPICAL RIDGE
NEAR THE NORTH-CENTRAL GULF COAST OVER THE NEXT 24-36 HOURS...
ACCOMPANIED BY A GRADUAL DECREASE IN THE FORWARD SPEED. AS THE
RIDGE BUILDS BACK IN TO THE NORTH OF THE CYCLONE...ISAAC IS
EXPECTED TO TURN A LITTLE MORE TOWARD THE WEST-NORTHWEST BEFORE IT
TURNS NORTHWARD ON DAY 4...AND MOVES NORTHEASTWARD INTO THE OHIO
VALLEY REGION ON DAY 5. THE OFFICIAL FORECAST TRACK HAS BEEN NUDGED
A LITTLE TO THE LEFT OF THE PREVIOUS ADVISORY TRACK AND LIES JUST
TO THE RIGHT OF THE CONSENSUS MODELS TV15 AND TCVA.
MID-LEVEL DRY AIR ENTRAINMENT CONTINUES TO PLAGUE ISAAC. AS SOON AS
THE CYCLONE DEVELOPS INNER-CORE CONVECTION AND THE APPEARANCE OF AN
EYE IN RADAR AND SATELLITE IMAGERY...THE CONVECTION ERODES DUE TO
THE DRY AIR. DESPITE THE FAVORABLE OCEANIC CONDITIONS AND
UPPER-LEVEL OUTFLOW REGIME...ISAAC ONLY HAS ABOUT 12-18 HOURS FOR
FURTHER STRENGTHENING TO OCCUR. THE OFFICIAL INTENSITY FORECAST IS
SIMILAR TO THE PREVIOUS ADVISORY...WITH ONLY TIMING ADJUSTMENTS
MADE TO THE INTENSITY FORECAST...AND CLOSELY FOLLOWS THE INTENSITY
MODEL CONSENSUS.
ISAAC IS A LARGE TROPICAL CYCLONE. A DANGEROUS STORM SURGE...HEAVY
RAINFALL...AND STRONG WINDS EXTEND WELL AWAY FROM THE CENTER AND
ARE EXPECTED TO AFFECT A LARGE PORTION OF THE NORTHERN GULF COAST.
FOR THIS REASON...IT IS IMPORTANT NOT TO FOCUS ON THE EXACT CENTER
LOCATION. THE THREAT OF HEAVY RAINFALL AND FLOODING IS ALSO
EXPECTED TO SPREAD INLAND OVER THE LOWER MISSISSIPPI VALLEY REGION
DURING THE NEXT FEW DAYS.
FORECAST POSITIONS AND MAX WINDS
INIT 28/1500Z 28.1N 88.5W 60 KT 70 MPH
12H 29/0000Z 28.8N 89.4W 70 KT 80 MPH
24H 29/1200Z 29.7N 90.4W 65 KT 75 MPH...INLAND
36H 30/0000Z 30.5N 91.5W 45 KT 50 MPH...INLAND
48H 30/1200Z 32.0N 92.0W 35 KT 40 MPH...INLAND
72H 31/1200Z 35.0N 92.5W 20 KT 25 MPH...INLAND
96H 01/1200Z 38.5N 90.5W 15 KT 15 MPH...POST-TROP/REMNT LOW
120H 02/1200Z 41.0N 86.0W 15 KT 15 MPH...POST-TROP/REMNT LOW
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FORECASTER STEWART
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